Bring 1 inch of water to boil in large pot. Set steamer rack in pot. Place potatoes, squash and garlic on rack. Cover pot and steam vegetables until tender, adding more water to pot if necessary, about 15 minutes.

Transfer vegetables to large bowl. Add butter and mash vegetables until fairly smooth. Season to taste with salt and pepper. Nutmeg optional.

Note: Can be prepared 2 hours ahead. Let stand at room temperature. Reheat uncovered in 350 degree oven for 30 minutes.